Lactic acid has been widely used in chemical, foodstuff and pharmaceutical industries. And more particularly, polylactic acid (PLA) will become an important substitute product for the petrochemical resources. With the improvement of the living standards, people have growing demands for resources, such as packaging materials (food boxes, food bags, agricultural plastics, etc.). According to statistics, the production only of the plastics and chemical fibers in the world has reached over 32 million ton/year at present. However, as the petroleum resources will gradually die out, some new renewable resources will become main substitute products, and polylactic acid is one of them. At present, PLA products (lunch boxes, medical appliances, etc.) have already been in popularization and application. Therefore, development and utilization of lactic acid as a renewal resource is of great significance, with broad market prospects.
In industry, lactic acid is mainly produced by the method of fermentation, using starch products as raw materials and subject to bacteria or rhizopus fermentation, i.e. fermentation→calcium salt neutralization→acid hydrolysis→lactic acid product. In the process, a great deal of calcium sulphate waste residue, waste water and carbon dioxide produce, causing serious environmental problems.
In order to reduce preparation costs and improve production quality of lactic acid, about it is reported at home and abroad about application of electrodialysis as the technology for extraction of fermentation broth. In the US patent “Process for the Purification of Lactic Acid” (U.S. Pat. No. 3,619,397) in 1971, there is descried a lactic acid extraction technology by firstly conducting solvent extraction and then employing electrodialysis with usual anionic membranes and cationic membrane stack. Toxic amine materials are generally used as the solvent for lactic acid extraction, which tends to cause secondary pollution to the product of lactic acid. In the Chinese patent “Extraction Process and Equipment for Lactic Acid Electrodialysis” (ZL 87104858.2), there is described calcium lactate extraction by electrodialysis composed of usual anionic membrane and cationic membrane stack, with conversion rate of 85%. However, this process still uses the method of calcium salt fermentation. Although calcium ions finally do not form calcium sulphate deposition, it will be discharged in other forms, causing serious environmental pollution. Denmark Plougmann, Vingtoft & Partners A/S has reported “Production of lactic acid from whey proteins where electrodialysis causes less biofouling of the electrodialysis membrane” (NZ336852), and has applied for patent protection of “Fermentative Production and Isolation of Lactic Acid” (ZL 97181922.X) in China. This invention applies a method of adding ammonia for formation of ammonium lactate, and isolating lactic acid by a process comprising ultra filtration, ion exchange, conventional electrodialysis and electrodialysis with bipolar membranes. This process causes certain environmental pollution due to severe volatilization. As all the above several methods do not practice comprehensive recycling of water for fermentation or discharge water, it is difficult to achieve high efficient utilization of water resources. Therefore, the cleaning process for lactic acid production has become the focus for domestic and foreign researches.